In the following description and in the appended claims, by the term ‘coolant’ a medium is designated that, when used in a cooling system, transports cold or heat substantially without change of phase between sites of differing temperature (though short-term local changes of phase in the coolant medium may occur). This holds, of course, for the operating conditions for which the system in question is designed, since a change of phase can always be brought about under sufficiently extreme conditions. Coolants are generally present in the liquid state. Water mixed with alcohol or with another antifreeze agent may, for example, be employed as coolant.
The term ‘refrigerant’, on the other hand, designates in the present case a medium that changes its state of aggregation in the course of the transport of cold or the transport of heat. As a rule, gaseous refrigerant is cooled down in a condenser and, in the process, passes over into the liquid state. By exchange of heat with a medium to be cooled, for example the coolant, the liquid refrigerant can evaporate and cool down the medium to be cooled by extracting therefrom the energy (enthalpy of evaporation) necessary for evaporating the refrigerant. This definition too relates to the conditions for which the system in which the refrigerant is being used is designed. Frequently CO2 or hydrocarbon compounds find application as a refrigerant.
Modern commercial aircraft are, for the most part, equipped with cooling systems, in order, for example, to make cooling functions available for galleys. The cooling systems should not be confused with the air-conditioning systems which are likewise present for regulating the air temperature and the atmospheric condition in the cabin and are, as a general rule, independent of these systems. In particular, larger aircraft frequently have more than one load with a cooling requirement; for example, several galleys may be present.
Hitherto this plurality of loads has frequently been supplied in decentralised manner. Each load was provided with a separate cooling unit or refrigerating machine assigned to it. Such a cooling unit is, as a rule, supplied in flight with cool ram air from a ram-air inlet. The ram air flows through a condenser and thereby cools a refrigerant. Via a customary refrigerant circuit the refrigerant is supplied to an evaporation device or evaporator in which a coolant is cooled by exchange of heat with the refrigerant. Therefore in conventional systems a plurality of different refrigerating machines or evaporation devices are necessary, which are matched to the relatively low cooling requirement of the individual loads. In conventional systems of such a type, waste heat is released into the fuselage of the aircraft, placing an additional burden on the air-conditioning system.
In modern commercial aircraft, however, use is increasingly being made of centralised aircraft cooling units. The various loads are supplied with cooled coolant, whereby in each instance one or more centralised evaporation devices are provided for cooling the coolant for several loads. In systems of such a type, the refrigerant with which the coolant is cooled is cooled by ram air, just as described above. However, in centralised systems fewer but larger ram-air inlets are provided, in order to ensure sufficient cooling.
As a rule, in centralised cooling systems of such a type two independent circuits for coolants of two refrigerating machines are cooled with refrigerant circuits, in order to ensure redundancy. Frequently in this case the evaporation devices are arranged one after the other in series in a refrigerant circuit.
WO 2005/030579 A1 describes an evaporator arrangement for an aircraft-galley cooling unit, in which four evaporators are employed. In each instance a pair of evaporators is flowed through by a stream of refrigerant. The two parallel arrangements are separate from one another, and each evaporator cools a separate stream of coolant.
DE 10 2006 022 557 A1 describes an ejector-pump circuit apparatus. According to this document, a stream of refrigerant is fed into different evaporators by passing through various throttle devices and an injection pump in various pressure states.
Given intense (cooling) loading of the coolant circuits, in particular given variably intense loading of the coolant circuits, the problem may arise that one of the coolant circuits requires so much cooling that the refrigerant has already passed over into the gas phase in such large proportions that a sufficient cooling of the following coolant circuit is no longer possible. It may even happen that an intensely loaded first coolant circuit completely evaporates the refrigerant in a first evaporator, so that virtually no cooling of the second circuit is possible any longer. A measurement and control of the stream of refrigerant at various places with several sensors and valves, intended to help prevent such states, is elaborate, costly and complex.
There is therefore a requirement for an aircraft cooling system evaporator arrangement that circumvents these problems and at the same time is simple and inexpensive in its manufacture.